The present invention concerns a thin-film transistor comprising a layer of semiconducting organic material provided on an insulating substrate and integrated with at least two electrical contacts for applying an electric current to said semiconducting organic material; a method for improving the chain order and increasing the conjugation length of a semiconducting organic polymer for use as a semiconducting material in a thin-film transistor; and a method for copolymerization of thiophenes, particularly thiophene and/or monomer thiophene compounds, in the manufacture of copolymer thiophene compounds with improved chain order and increased conjugation length for use in organic thin-film transistors. The invention also concerns the use of a semiconducting copolymer in an organic thin-film transistor as well as the use of a copolymer of thiophene compounds in an organic thin-film transistor.
Thin-film transistors have lately been the subject of increasing attention with regard to the fabrication of low-cost and flexible displays and other uses within the electronics where it is desired to manufacture components and devices covering large areas. Inexpensive semiconducting polymer materials which are soluble, easy to process and environmentally stable offer important advantages in such applications. Homopolymers of 3-alkyl thiophenes with alkyl groups equal to or greater than butyl in size are easily soluble at room temperature in common solvents such as chloroform, toluene, xylene, tetrahydrofuran etc. However, the incorporation of a large alkyl side group also will increase the distance between the xcfx80-conjugated backbones and hence reduce the effective conjugation length. For organic thin-film transistors a large effective conjugation length in the active organic semiconductor is directly related to high carrier mobility. The carrier mobility of solvent cast poly(3-alkyl thiophene) typically lies in the range 10xe2x88x928-103 cm2/Vxc2x7s, such as evident from F. Garnier, xe2x80x9cThin-Film Transistors Based On Organic Conjugated Semiconductorsxe2x80x9d, Chemical Physics, 227:253-262 (1998).
Regioregular poly(3-hexyl thiophenes) P3HT) which are characterized as polymers with regiospecificity from head-to-tail, have a significantly extended conjugation length and highly ordered structures such as evident from T-A. Chen, X. Wu and R. D. Rieke, xe2x80x9cRegiocontrolled Synthesis of Poly(3-alkyl thiophenes)xe2x80x9d, mediated by Riek Zinc, xe2x80x9cTheir Characterization and Solid-State Propertiesxe2x80x9d, J.Am. Chem. Soc., 117: 233-244 (1995).
Higher mobility, viz. 0.01-0.4 cm2/Vxc2x7s, has been described by Zenan Bao, Yi Feng, Ananth Dodabalapur, V.R. Raju and Andrew J. Lovinger, Chemistry of Materials, vol. 9: 1299-1301 (1997) for regioregular poly(3-hexyl thiophene). However, the complicated regiosynthesis and purification procedures significantly increase the material cost. It has therefore been necessary to develop other types of poly(3-alkyl thiophenes) which have a moderate synthesis procedure and which offer high carrier mobility for organic thin-film transistors. A method for synthesizing poly(3-alkyl thiophenes) is described in R-I. Sugimoto, S. Takeda, H. B. Gu and K. Yoshino, xe2x80x9cPreparation of Soluble Polythiophene Derivates Utilizing Transition Metal Halides as Catalysts and Their Propertyxe2x80x9d, Chemistry Express, Vol. 1, No. 11:635-638 (1986).
A first object of the present invention is hence to provide a thin-film transistor with an organic semiconducting material offering a higher carrier mobility. Another object of the present invention is to develop new types of polymers for the semiconducting material. Particularly it is an object of the invention to provide polymers of this kind which can be processed in solution and which have a higher order of chain structure, but simultaneously lower cost. It is also an object of the invention to increase the conjugation length of such polymers. Finally it is an object of the present invention to provide semiconducting polymer materials for use in organic thin-film transistors.
The above-mentioned objects and other features and advantages are obtained according to the present invention with thin-film transistor which is characterized in that said semiconducting organic material is a semiconducting organic copolymer with a highly ordered chain structure and increased conjugation length and formed from at least two organic monomer compounds; and with a method characterized by forming a copolymer from at least two organic monomer compounds.
In a first advantageous embodiment according to the invention the semiconducting organic copolymer is a copolymer of thiophenes, particularly thiophene and/or alkyl-substituted monomer thiophene compounds.
In this connection it preferred that the copolymer is either a copolymer of thiophene and one or more alkyl-substituted thiophene monomers, or a copolymer of thiophene and one of the following thiophene compounds: 3-hexylthiophene, 3-butylthiophene, or dodecylthiophene, or a copolymer of 3-methylthiophene and one of the following thiophene compounds: 3-hexylthiophene, 3-butylthiophene, or dodecylthiophene.
In a second advantageous embodiment of the transistor according to the invention the copolymer is a copolyiner of one or more of the following monomer compounds: substituted and/or unsubstituted phenylenes, xylylenes, xylylidenes, phenylene sulfides, or classes of these monomer compounds.
In a third advantageous embodiment of the transistor according to the invention the copolymer is a copolymer of general semiconducting and/or conducting organic polymers synthesized from monomer compounds. Particularly it is in this connection preferred that the organic polymers are synthesized from one of the following compounds: N-vinylcarbazole, epoxyproprycarbazole, pyromellitimide, N-phenylbenzimidazole, benzoxazole, acylonitrile, azine, phtalocyanine, vinylene, azophenylene, phenylenequinone, or phenyleneferrocene.
In a first advantageous embodiment of the method according to the invention the copolymer is formed with different fractions of said at least two monomer compounds, the fraction of a smaller monomer compound being lower than that of a larger monomer compound.
In a second advantageous embodiment the method according to the invention the monomer compounds are selected among asymmetrical monomer compounds.
In a third advantageous embodiment of the method according to the invention, monomer units or molecule sections of monomers having a minimal or reduced steric hindrance are incorporated in the copolymer.
Other advantageous embodiments of the above method according to the invention are apparent from the appended dependent claims 13-19 which disclose various preferred compounds used for forming the copolymer, these compounds being similar to those disclosed for the copolymer as used in the various advantageous embodiments of the transistor according to the invention as stated hereinbefore.
Particularly the above mentioned objects and other feature and advantages are obtained according to the present invention with a further method which is characterized by comprising steps for mixing thiophene or a first monomer thiophene compound with a second monomer thiophene compound, such that said compounds form a monomer mixture with a molar feed ratio of the components thiophene or said first monomer thiophene compound and said second monomer thiophene compounds between 0:10 and 4:6, forming a reaction mixture of said monomer mixture by adding the latter to a stirred solution of an oxidant, the solution being formed with a solvent purged with an inert gas, such that a reaction mixture is obtained, stirring said reaction mixture during a first time period at a given temperature, adding said reaction mixture to precipitation agent such that a precipitate is formed, rinsing said precipitate with a rinsing agent, such that a solid substance is obtained and subjecting said solid substance to extraction with a first extraction agent during a second given time period and thereafter with a second extraction agent during a given third time period, whereby a purified reaction product in the form of poly(3-alkyl thiophene) was formed and thereafter dried under vacuum.
Various advantageous embodiments of the further method according to the invention are apparent from the appended dependent claims 21-34.
Finally the above-mentioned and other features and advantages are obtained according to the invention with the use of a semiconducting organic copolymer with a highly ordered chain structure and increased conjugation length and formed from at least two organic monomer compounds in an organic thin-film transistor, and particularly with the use of a copolymer of 3-methyl thiophene and 3-hexyl thiophene as an active semiconducting material in an organic thin-film transistor.